In solid state polymerization, or xe2x80x9cSSPxe2x80x9d, a high molecular weight polymer is produced by first preparing a relatively low molecular weight crystallized prepolymer followed by its conversion to a higher molecular weight material in the solid state. A solid phase polymerization process is made possible by the ability of both the starting molecular weight material and the product high polymer to sustain temperatures above the glass transition temperature without fusion of the polymer. The temperature at which the process is conducted must be sufficient to effect chain growth and the attendant increase in Mw.
Solid state polymerization (SSP) of polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073. Typically, SSP involves three steps: a first step of forming a prepolymer, typically by melt polymerization (i.e. transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate, such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. The second or crystallization step as disclosed in these patents is by solvent treatment or heat treatment.
There exists a need for an improved method for crystallizing the polycarbonate prepolymer.
There exists a further need for a prepolymer in a form which may be readily processed to produce polycarbonate having improved properties.
There exists a further need for a process in which the in which the rate of solid state polymerization may be effectively increased.
There further exists a need for a SSP process which may be conducted at shorter processing times.
There further exists a need for a SSP process in which the process may be conducted in fewer stages.
In a first aspect, the invention relates to a method of crystallizing an aromatic polycarbonate prepolymer having a weight average molecular weight of from about 1,000 to about 20,000 and having from about 5 to about 95 mole % aryl carbonate terminal end groups, based on total end groups, the method comprising:
a) contacting the prepolymer with a crystallization agent comprising an alcohol or alcohols under conditions effective to crystallize the prepolymer.
Optionally, the crystallization agent may comprise an additive. The crystallization agent preferably comprises at least 95% by weight of the alcohol or alcohols.
In one embodiment of this aspect of the invention, the invention relates to a method of crystallizing an aromatic polycarbonate prepolymer having a weight average molecular weight of from about 1,000 to about 20,000 and having from about 5 to about 95 mole % aryl carbonate terminal end groups, based on total end groups, the method comprising:
a) contacting the prepolymer with an agent comprising a secondary alcohol, wherein the secondary alcohol is applied to the prepolymer in the vapor phase.
In a further embodiment, this aspect of the invention relates to a method a method of increasing the rate of solid state polymerization during preparation of an aromatic polycarbonate from a prepolymer, the method comprising:
a) contacting the prepolymer with a crystallizing agent comprising an alcohol and an additive, the additive comprising a substance effective to increase the rate of solid state polymerization.
In a further embodiment, this aspect of the invention relates to a method of controlling the thermal crystallization of a prepolymer having a weight average molecular weight of from about 1,000 to about 20,000 and having from about 5 to about 95 mole % aryl carbonate terminal end groups, based on total end groups, the method comprising the step of:
a) applying a crystallizing agent comprising an alcohol or alcohols to the prepolymer.
Optionally, the crystallization agent may comprise an additive. The crystallization agent preferably comprises at least about 95% by weight of the alcohol or alcohols.
In a second aspect, the invention relates to a method of preparing cellular pellets from a prepolymer comprising a blowing agent, the method comprising the steps of:
a) extruding the prepolymer through a die, the die maintained at conditions such that the blowing agent remains in the condensed phase in the prepolymer prior to emerging from the die, and
b) upon emergence of the prepolymer through the die, substantially simultaneously cooling the prepolymer by contacting the prepolymer with a cooling agent and cutting the prepolymer; the conditions outside the die being maintained such that the blowing agent vaporizes in the prepolymer to form pores.
In one embodiment, the second aspect of the invention relates to a method of preparing cellular pellets comprising an aromatic polycarbonate prepolymer having a weight average molecular weight of from about 1,000 to about 20,000 and having from about 5 to about 95 mole % aryl carbonate terminal end groups, based on total end groups, the aromatic polycarbonate prepolymer further comprising a blowing agent, the method comprising:
a) extruding the aromatic polycarbonate prepolymer through a die, the die maintained at conditions such that the blowing agent remains in the condensed phase prior to emerging from the die, and
b) upon emergence of the aromatic polycarbonate prepolymer through the die, substantially simultaneously cooling the aromatic polycarbonate prepolymer by contacting the aromatic polycarbonate prepolymer with a cooling agent and cutting the aromatic polycarbonate prepolymer; the conditions outside the die being maintained such that the blowing agent vaporizes in the aromatic polycarbonate prepolymer to form pores.
In a further embodiment of this second aspect of the invention, the cellular pellets are crystallized by effecting contact with an agent comprising an alcohol and further subjected to a process of solid state polymerization.
Additional advantages of the invention will be set forth in the description which follows. The advantages of the invention will be realized and attained by means of the elements and combinations set forth in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary only, and not restrictive of the invention, as claimed.